This invention relates generally to front-end radio frequency filters including film bulk acoustic resonators (FBAR).
Film bulk acoustic resonators have many advantages compared to other techniques such as surface acoustic wave (SAW) devices and ceramic filters, particularly at high frequencies. For example, SAW filters begin to have excessive insertion losses above 2.4 gigahertz and ceramic filters are much larger in size and become increasingly difficult to fabricate at increased frequencies.
A conventional FBAR filter may include two sets of FBARs to achieve the desired filter response. The series FBARs have one frequency and the shunt FBARs have another frequency. The frequency of an FBAR is mainly determined by the thickness of its piezoelectric film which approximately equals the half wavelength of the acoustic wave. The frequencies of the FBARs need to be precisely set to achieve the desired filter response.
For example, for a 2 gigahertz FBAR, the thickness of the piezoelectric film may be approximately 1.8 micrometers. A one percent non-uniformity in piezoelectric film thickness may shift the frequency of the filter by approximately 20 megahertz which is not acceptable if a 60 megahertz pass bandwidth is required.
Generally, post-process trimming may be used to correct the frequency. One technique may involve etching the upper electrode or depositing more metal. Another technique involves adding a heating element. However, both of these approaches are problematic in high volume manufacturing, particularly since they are die-level processes that generally have low throughput. In addition, in-situ measurement may be required during the post-process trimming steps. Therefore, the costs are high and the throughput is relatively low.
Thus, there is a need for better ways to adjust the frequency of FBARs.